1. Field of the Invention
This invention relates generally to sheet feeding apparatus, and more particularly to a timing mechanism for a sheet feeder which assures the feeding of a sheet from the top of a supply stack at a particular time, depending upon the height of the stack, whereby said sheets are received at a sheet utilization device during a predetermined portion of the operative cycle of the sheet utilization device.
2. Description of the Prior Art
In the field of printing or high speed copy/duplicating, it is common practice to feed discrete copy receiving sheets to the printing station from a stack of sheets supported in a supply hopper. Feeding of the sheets may be accomplished either pneumatically or mechanically. During their transport between the supply hopper and the printing station, the sheets usually pass through a registration station to insure proper synchronization and alignment of the sheet with the arrival of an image to be copied in the printinng station. As the operating speed of the modern printing apparatus has increased, the available period of time for transport and registration of copy receiving sheets has decreased. Moreover, the importance of feeding single sheets (i.e., prevention of multiple sheet feeds) has increased.
One type of sheet feed apparatus capable of high speed operation for feeding of single discrete sheets from a supply stack is the well known scuff feeder. Scuff feeders, comprising friction rollers or pads, are generally located above the stack of sheets so that the weight of the feeder (or perhaps the force from an additional urging mechanism) provides sufficient frictional force with the topmost sheet to feed discrete sheets one at a time from the stack. If the stack of sheets is supported on a fixed base, the length of the feed path from the stack to the printing station (or the registration station) changes with changes in the height of the stack. While in the instance of slower speed printing apparatus the longer feed path may be inconsequential in the operation cycle, in the the case of high speed apparatus, the length of time a sheet remains in the longer feed path may exceed the operation cycle time required for feed and registration of the sheet prior to printing. One way to insure adequate feed and registration in high speed printing apparatus is to maintain a constant length sheet feed path. To achieve a constant length feed path, the scuff feeder is generally supported in a fixed position relative to the printing station and the supply stack is supported on an adjustable platform which is periodically raised to insure contact of the topmost sheet in the stack with the feeder (as for example, U.S. Pat. Nos. 3,995,952; 4,008,957). However, elevator mechanisms in general use for raising the stack supporting platform to maintain the topmost sheet of the stack at a particular position relative to the scuff feeder to insure accurate single sheet feeding are of relatively complicated construction.